1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus including a reflection display unit and a transmission display unit within one pixel.
2. Description of the Prior Art
One type of the reflection-type liquid crystal display apparatuses has a reflection display unit and a transmission display unit within one pixel. The reflection display unit performs the display by reflecting, using a reflection plate, the light incoming from the surroundings. Since the reflection display unit has a fixed contrast ratio independently of the brightness of the surroundings, it offers an advantage of making it possible to obtain an excellent display under an environment ranging from outdoors to indoors at the time of sunny weather. The reflection display unit, however, becomes incapable of recognizing the display in a dark environment such as a dark room.
Meanwhile, the transmission display unit performs the display by taking advantage of the light from a backlight light-source located at the rear thereof. The transmission display unit offers an advantage of making it possible to recognize the display even in the dark environment such as the dark room. The transmission display unit, however, becomes incapable of recognizing the display under a bright environment where, e.g., the intensity of the interfacial reflection light is stronger than that of the backlight light.
In this way, the reflection display unit and the transmission display unit are in a relationship of complementing the respective disadvantages to each other. As a result, the reflection-type liquid crystal display apparatus including both of them is usable under a wider condition including the outdoors at the time of sunny weather to the dark room. The reflection-type liquid crystal display apparatus including the reflection display unit and the transmission display unit within one pixel has been described in, e.g., JP-A-11-242226.
In the reflection-type liquid crystal display apparatus including the reflection display unit and the transmission display unit (hereinafter, referred to as simply xe2x80x9cthe reflection-type liquid crystal display apparatusxe2x80x9d), the reflection display unit has built the reflection plate in a liquid crystal cell, and includes one sheet of polarizer and one sheet or two sheets of phase plates. Meanwhile, the transmission display unit in the reflection-type liquid crystal display apparatus uses one sheet of polarizer over and under a liquid crystal cell, and includes one sheet or two sheets of phase plates between the liquid crystal cell and the polarizers.
The display characteristics of the reflection display unit are determined by the respective optical parameters of the liquid crystal layer, the phase plates, and the polarizers. The optical parameters of the liquid crystal layer are a twist angle and a retardation. The optical parameters of the phase plates are the azimuthal angle of a slow axis and a retardation. The optical parameter of the polarizers is the azimuth-angle of an absorption axis.
Similarly, the display characteristics of the transmission display unit are also determined by the optical parameters of the liquid crystal layer, the phase plates, and the polarization plates.
On the upper side of the reflection-type liquid crystal display apparatus (i.e., on an observation-plane side of the liquid crystal display apparatus), the phase plate and the polarizer located thereon can be used in common to the reflection display unit and the transmission display unit. On the other hand, on the lower side of the reflection-type liquid crystal display apparatus, it is difficult to form, as one and the same layer, a reflection electrode for applying a voltage to the reflection display unit and a transparent electrode for applying a voltage to the transmission display unit. Accordingly, both of the electrodes are usually formed as different layers. This condition results in a difference in the liquid crystal layer thickness between the reflection display unit and the transmission display unit, thereby making it impossible to obtain excellent displays on both the reflection display and the transmission display.
On account of this, in a reflection-type liquid crystal display apparatus using, e.g., a super-twisted nematic liquid crystal, the margin for a variation in the liquid crystal layer thickness is extremely narrow. Consequently, a configuration has been employed where layer-gap between the reflection display unit and the transmission display unit is eliminated. In this apparatus, however, the combination of a lower-side phase plate and a lower-side polarizer has been formed into an elliptical polarizer in order to increase the transmission of the transmission display unit. In this case, in spite of the condition that there exists no layer-gap between the reflection display unit and the transmission display unit, the combination has been formed into the elliptical polarization plate. This reduces the contrast ratio on the transmission display down to an order of 10:1.
The present invention has been made in view of these problems, and provides a reflection-type liquid crystal display apparatus that allows the excellent displays to be obtained on both the reflection display and the transmission display.
In order to solve the above-described problems, the present invention has employed the following method:
In a liquid crystal display apparatus including a pair of facing substrates, a liquid crystal layer and a liquid crystal driving unit which are held in being sandwiched between the facing substrates, and polarization plates and phase plates which are located on the upper side and on the lower side of the facing substrates, respectively, wherein a pixel of the liquid crystal display apparatus includes a reflection display unit whose reflectivity""s applied voltage dependance of reflection is the normally-closed type and a transmission display unit whose layer thickness is thicker than that of a liquid crystal layer constituting the reflection display unit. Moreover, the polarization plate and the phase plate which are located on the lower side of the facing substrates form an elliptical polarizer, thereby converting, into a circularly-polarized light, a backlight light at a point-in-time of having passed through a liquid crystal layer""s portion corresponding to a difference in the layer thickness between the liquid crystal layers.
An optical parameter setting method for the liquid crystal layer, the phase plates, and the polarizer of the reflection-type liquid crystal display apparatus has been described in, e.g., a presentation given by O. Itou, S. Komura, K. Kuwabara, K. Funahata, K. Kondo, K. Kubo et al. (SID ""98 DIGEST (1998), pp. 766-769). In the reflection-type liquid crystal display apparatus, a light is incident into the polarizer, and passes through the phase plate and the liquid crystal layer, then being reflected by the reflection plate. Moreover, the light passes through the liquid crystal layer and the phase plate again, then being incident into the polarizer. At the time of the dark display, if, in this process, a phase difference equivalent to a half wavelength is added to the light, the light is completely absorbed at a point-in-time of being incident into the polarizer at the second time, which, accordingly, is ideal. Namely, this is because the oscillation plane of the light that has become a linearly-polarized light by passing through the polarizer at the first time rotates by 90xc2x0 in this process, and thus the oscillation direction becomes parallel to an absorption axis of the polarizer at the point-in-time of being incident into the polarizer at the second time. When performing the conversion into the one-way light path, the phase difference added to the transmission light is equal to a quarter wavelength. Accordingly, at a point-in-time of having reached the reflection plate, the polarization state of the transmission light becomes a circularly-polarized light.
Also, at the time of the bright display, if a phase difference equivalent to the one wavelength is added to the light, the light passes through completely at the point-in-time of being incident into the polarizer at the second time, which, accordingly, is ideal. Namely, this is because, at this time, the oscillation plane of the light that has become the linearly-polarized light in passing through the polarizer at the first time does not rotate, and thus the oscillation direction becomes perpendicular to the absorption axis of the polarization at the point-in-time of being incident into the polarization at the second time. When performing the conversion into the one-way light path, the phase difference added to the transmission light is equal to a half wavelength. Accordingly, at the point-in-time of having reached the reflection plate, the polarization state of the transmission light becomes a linearly-polarized light.
FIG. 1 and FIG. 2 illustrate the above-described situation, using the Poincarxc3xa9 sphere display. Incidentally, the Poincarxc3xa9 sphere is a sphere that is 1 in radius and is defined within a space the three axes of which are Stokes parameters S1, S2, and S3. As illustrated in FIG. 17, the respective points on the sphere are in a one-to-one correspondence with all the polarization states. On the Poincarxc3xa9 sphere display, e.g., a sectional line with the (S1, S2) plane corresponds to a linearly-polarized light, and an intersection point with the S3 axis corresponds to a circularly-polarized light. The other portions correspond to elliptically-polarized lights.
FIG. 1 illustrates the ideal polarization conversion on the dark display. The explanation will be given below concerning FIG. 1. The transmission light after having passed through the polarizer has become the linearly-polarized light, thus being positioned at a point L1 on the equator on the Poincarxc3xa9 sphere. Then, the transmission light, after having passed through the phase plate and the liquid crystal layer, is converted into the circularly-polarized light. Namely, the transmission light rotates on the Poincarxc3xa9 sphere by a quarter rotation, thereby moving to a pole P. After having been reflected, the transmission light passes through the liquid crystal layer and the phase plate again, thereby becoming the linearly-polarized light the oscillation direction of which has rotated by 90xc2x0. Namely, the transmission light rotates on the Poincarxc3xa9 sphere by a one-fourth rotation again, thereby moving to a point L2 situated on the opposite side of the point L1 on the equator.
FIG. 2 illustrates the ideal polarization conversion on the bright display. The transmission light after having passed through the polarizer has become the linearly-polarized light, thus being positioned at the point L1 on the equator on the Poincarxc3xa9 sphere. Then, the transmission light, after having passed through the phase plate and the liquid crystal layer, is converted into the linearly-polarized light the oscillation direction of which has rotated by 90xc2x0. Namely, the transmission light rotates on the Poincarxc3xa9 sphere by a half rotation, thereby moving to the point L2 situated on the opposite side of the point L1 on the equator. After having been reflected, the transmission light passes through the liquid crystal layer and the phase plate again, thereby becoming the original linearly-polarized light. Namely, the transmission light rotates on the Poincarxc3xa9 sphere by a half rotation again, thereby returning back to L1.
Although the constant ratio is expressed by a ratio in the reflection between the time of the bright display and the time of the dark display, what exerts an influence upon the constant ratio mainly is the reflection at the time of the dark display. The optical parameters of the liquid crystal layer, the phase plates, and the polarizers are set so that, on the dark display, the polarization conversion as described above can hold in the entire visible wavelength band. Based on this setting, an upper-side polarizer, an upper-side phase plate, and the liquid crystal layer thickness of the reflection display unit are determined.
Next, the explanation will be given below concerning the ideal polarization conversion in the transmission display unit and a lower-side polarization plate and a lower-side phase plate implementing this polarization conversion. In the case where there exists no layer gap between the reflection display unit and the transmission display unit, if a backlight light is incident into the liquid crystal layer in a state of having become a circularly-polarized light, the ideal dark display can be obtained. Namely, at the time of the dark display, the liquid crystal layer and the upper-side phase plate add a phase difference of a quarter wavelength to a light passing therethrough. Consequently, if the circularly-polarized light is incident therein, it is converted into a linearly-polarized light, then being completely absorbed by the upper-side polarizer. In order to convert the backlight light into the circularly-polarized light, it is advisable to make the lower-side phase plate a quater wavelength plate, and to locate a transmission axis of the lower-side polarizer in such a manner as to form 45xc2x0 toward a slow axis of the lower-side phase plate.
FIG. 3 illustrates the above-described situation, using the Poincarxc3xa9 sphere notation. FIG. 3 illustrates a Poincarxc3xa9 sphere display of the transmission display unit in the case where there exists no layer gap between the reflection display unit and the transmission display unit. Accordingly, FIG. 3 illustrates the ideal polarization conversion on the dark display. The transmission light after having passed through the polarizer has become the linearly-polarized light, thus being positioned at a point L1 on the equator on the Poincarxc3xa9 sphere. Then, the transmission light, after having passed through the polarizer and the lower-side phase plate, is converted into the circularly-polarized light, thereby moving to a pole P. Moreover, after having passed through the first liquid crystal layer, the transmission light passes through the upper-side phase plate, thereby becoming the linearly-polarized light the oscillation direction of which has rotated by 90xc2x0 and moving to a point L2.
On the Poincarxc3xa9 sphere notation, the conversion of the polarization state by the phase plates is expressed as a rotation around a rotation axis existing within the (S1, S2) plane and penetrating the center of the Poincarxc3xa9 sphere. This rotation axis is equivalent to the slow axis of the phase plates. In FIG. 3, the rotation axis SA indicating the slow axis of the phase plates is expressed by the dashed line. Assuming that an angle formed between the transmission axis of the lower-side polarizer and the slow axis of the lower-side phase plate is equal to xcex8, an angle formed between L1 and the rotation axis is equal to 2xcex8. In FIG. 3, since the angle formed between the transmission axis of the lower-side polarizer and the slow axis of the lower-side phase plate is equal to 45xc2x0, the angle formed between L1 and the rotation axis is equal to 90xc2x0.
Meanwhile, in the case where there exists a layer gap between the reflection display unit and the transmission display unit, and the transmission display unit is thicker in the liquid crystal layer thickness, let""s consider dividing the liquid crystal layer of the transmission display unit into two portions with reference to the thickness direction. Considering, as a boundary, a lower-side interfacial plane of the liquid crystal layer of the reflection display unit, the side lower than this is assumed to be a layer-gap unit liquid crystal layer, and the side upper than this is assumed to be a reflection display unit liquid crystal layer.
At a point-in-time when the backlight light has passed through the layer-gap unit liquid crystal layer and has reached the boundary, if the backlight light has become a circularly-polarized light, the requirements will be satisfied. Namely, in order that the layer-gap unit liquid crystal layer will also add the phase difference to the backlight light, the combination of the lower-side phase plate and the layer-gap unit liquid crystal layer is required to operate similarly to a quarter wavelength plate. At this time, at a point-in-time of having passed through the lower-side phase plate, the backlight light becomes an elliptically-polarized light.
FIG. 4 illustrates the above-described situation, using the Poincarxc3xa9 sphere display. FIG. 4 illustrates a Poincarxc3xa9 sphere notation of the transmission display unit in the case where the transmission display unit is thicker in the liquid crystal layer thickness. Accordingly, FIG. 4 illustrates the ideal polarization conversion on that dark display. The transmission light after having passed through the lower-side polarizer has become a linearly-polarized light, thus being positioned at a point L1 on the equator on the Poincarxc3xa9 sphere. Then, the transmission light, after having passed through the polarizer and the lower-side phase plate, is converted into the elliptically-polarized light E. Moreover, after having passed through the layer-gap unit liquid crystal layer, the transmission light is converted into the circularly-polarized light, thereby moving to a pole P. In addition, the transmission light passes through the reflection display unit liquid crystal layer and the upper-side phase plate, thereby becoming a linearly-polarized light the oscillation direction of which has rotated by 90xc2x0 and moving to a point L2 on the equator.
The movement from the elliptically-polarized light E to the pole P is an operation caused by the layer-gap unit liquid crystal layer, and a rotation angle from the elliptically-polarized light E to the pole P is denoted by "psgr". In order to convert the backlight light into the elliptically-polarized light E after the backlight light has passed through the lower-side phase plate, an angle formed between the rotation axis and L1 must be made equal to 90xc2x0 or less on the Poincarxc3xa9 sphere. An angle by the amount of the shifting from 90xc2x0 of the angle formed between the rotation axis and L1 is equal to 90xc2x0xe2x88x922xcex8. As is clearly seen from FIG. 4, "psgr" is equal to 90xc2x0xe2x88x922xcex8. Also, letting a substantial birefringence of the liquid crystal layer be xcex94n, the layer-gap between the reflection display unit and the transmission display unit be d, and a wavelength of the backlight light be xcex, "psgr" is represented by the following equation:
"psgr"=360xc2x0xc3x97xcex94nd/xcexxe2x80x83xe2x80x83(1)
Also, from the condition that "psgr" is equal to 90xc2x0xe2x88x922xcex8, the following equation can be obtained:
45xc2x0xe2x88x92=180xc2x0xc3x97xcex94nd/xcexxe2x80x83xe2x80x83(2)
In this way, the mutual connection has been established between d, i.e., the layer-gap between the reflection display unit and the transmission display unit, and xcex8, i.e., the angle formed between the lower-side polarizer absorption axis and the lower-side phase plate slow axis.
Namely, from the equation (2), even if the layer-gap d between the reflection display unit and the transmission display unit is varied, xcex8 is adjusted in accordance with the equation (2). This makes it possible to reduce the dark display transmission on the transmission display. Also, in the case where the layer-gap d between the reflection display unit and the transmission display unit is varied, the lower-side phase plate need not be replaced by the other type of phase plate with a different retardation. Rather, it is advisable enough to change the pasting angle of the lower-side phase plate in accordance with the equation (2).
Also, in correspondence with the layer-gap d between the reflection display unit and the transmission display unit, xcex8 is set in accordance with the equation (2). This makes it possible to convert, into the circularly-polarized light, the backlight light at the point-in-time of having passed through layer-gap liquid crystal layer. At this time, if the upper-side polarizer and the upper-side phase plate have been set in advance so that a light having passed through the upper-side polarizer, the upper-side phase plate, and the first liquid crystal layer becomes a circularly-polarized light, it becomes possible to sufficiently reduce the dark display transmittance on the transmission display, thereby allowing a high contrast ratio transmission display to be obtained. Also, if the light having passed through the upper-side polarizer, the upper-side phase plate, and the first liquid crystal layer has become the circularly-polarized light, it becomes possible to sufficiently reduce the dark display reflection on the reflection display. In this way, the high contrast ratio transmission display and the high contrast ratio reflection display can be obtained simultaneously. What is more, the reflection in the reflection display unit and the transmissivity in the transmission display unit can be reduced sufficiently in one and the same applied voltage.
By the way, the above-described lower-side phase plate can be configured with two sheets of phase plates. The two sheets of phase plates are referred to as a first lower-side phase plate and a second lower-side phase plate from the one nearer to the liquid crystal layer. It has been known the following: In general, the combination of a quarter wavelength plate and a half wavelength plate makes it possible to form a circular polarization plate with a broad-band, thereby allowing the transmission lights in the entire visible wavelength range to be converted into polarized lights closer to circularly-polarized lights. As is the case with this, the retardation of the first lower-side phase plate and that of the second lower-side phase plate are set to be a quarter wavelength and a half wavelength, respectively. An elliptical polarizer formed as the result of this allows the transmission lights in the entire visible wavelength range to be converted into elliptically-polarized lights that are more identical to each other, thereby making it possible to obtain the transmission lights with an even higher contrast ratio.